Hematopoiesis, or the formation of blood cells, begins in the developing human embryo as clusters of stem cells. Although the spleen, lymph nodes and bone marrow all make small contributions to blood cell development, not until the fourth month does the bone marrow become the principal site of hematopoiesis.
In the adult, hematopoiesis involves the bone marrow, the lymph nodes and the spleen. These organs and associated tissues are traditionally divided into myeloid and lymphoid tissue-types. The myeloid/lymphoid division is somewhat artificial as these two types of tissues are believed to originate from a single pluripotent stem cell.
Lymphoid and myeloid stem cells, formed from division of the pluripotent cell, are precursors for all subsequent cell types. The bone marrow provides a unique environment for pluripotent and committed cells.
Hemoglobin comprises four protein chains, two alpha chains and two beta chains (α2β2), interwoven together, each with its own molecule of iron and with a combined molecular weight of about 68 kD. The hemoglobin macromolecule is normally glycosylated and upon absorbing oxygen from the lungs transforms into oxyhemoglobin (HbO2). There are at least six distinct forms of hemoglobin, each expressed at various times during development. Hemoglobin in the embryo is found in at least three forms, Hb-Gower 1 (ζ2ε2), Hb-Gower 2 (α2ε2), and Hb-Portand (ζ2γ2). Hemoglobin in the fetus comprises nearly totally HbF (α2γ2), whereas hemoglobin in the adult contains about 96% HbA (α2β2), about 3% HbA2 (α2δ2) and about 1% fetal HbF (α2γ2). The embryonic switch of globin expression from ζ to α and from ε to γ begins in the yolk sac. However, chains of embryonic ζ and ε have been found in the fetal liver and complete transition to the fetal form does not occur until late in fetal development. The fetal switch from γ to β begins later in erythropoiesis with the amount of γ globin produced increasing throughout gestation. At birth, β globin accounts for about 40% of non-α globin chain synthesis and thereafter continues to rapidly increase. Neither the switch from embryonic to fetal or fetal to adult appears to be controlled through cell surface or known cytokine interactions. Control seems to reside in a developmental clock with the switch occurring at times determined only by the stage of fetal development.
Defects or mutations in globin chain expression are common. Some of these genetic mutations pose no adverse or only minor consequences to the person, however, most mutations prevent the formation of an intact or normal hemoglobin molecule through a functional or structural inability to effectively bind iron, an inability of the chains or chain pairs to effectively or properly interact, an inability of the molecule to absorb or release oxygen, a failure to express sufficient quantities of one or more globin chains or a combination of these malfunctions.
Individuals with severe sickle cell anemia develop no symptoms until about five to six months after birth. In these infants it was determined that fetal hemoglobin did not interact with HbS and, as long as sufficient quantities were present, could modulate the effects of HbS disease. This modulating effect of β globin is also observed with other β globin disorders, such as HbC and HbD, and other mutations of the β chain. HbS polymerization is also significantly affected by the hemoglobin concentration of the cell. The higher the HbS concentration, the greater the chances for contact between two or more HbS molecules. Dehydration increases hemoglobin concentration and greatly facilitates sickling.
The thalassemia syndromes are a heterogenous group of disorders all characterized by a lack of or a decreased synthesis of the globin chains of HbA. Deficiencies of β-globin expression are referred to as β-thalassemias and deficiencies of α-globin, α-thalassemias. The hemolytic consequences of deficient globin chain synthesis result from decreased synthesis of one chain and also an excess of the complementary chain. Free chains tend to aggregate into insoluble inclusions within erythrocytes causing premature destruction of maturing erythrocytes and their precursors, ineffective erythropoiesis, and the hemolysis of mature red blood cells. The underlying defects of hemoglobin synthesis have been elucidated over the years and largely reside in the nucleic acid sequences which express or control the expression of a or β globin protein.
A variety of small molecules have been shown to effect hemoglobin or fetal globin expression. Early experiments demonstrated that acetate (CH3COOH), propionate (CH3CH2COOH), butyrate (CH3CH2CH2COOH) and isobutyrate (CH3CH(CH3)COOH) all induced hemoglobin synthesis in cultured Friend leukemia cells (E. Takahashi et al., Gann 66:577-80, 1977). Additional studies showed that polar compounds, such as acid amides, and fatty acids could stimulate the expression of both fetal and adult globin genes in murine erythroleukemia cells (U. Nudel et al., Proc. Natl. Acad. Sci. USA 74:1100-4, 1977). Hydroxyurea (H2NCONHOH), another relatively small molecule, was found to stimulate globin expression (N. L. Letvin et al., N. Engl. J. Med. 310:869-73, 1984). Stimulation, however, did not appear to be very specific to fetal globin (S. Charache et al., Blood 69:109-16, 1987). Hydroxyurea is also a well-known carcinogen making its widespread and long term use as a pharmaceutical impractical.
One of the major breakthroughs in the treatment of hemoglobinopathies was made when it was discovered that butyric acid (butanoic acid; CH3CH2CH2COOH) accurately and specifically stimulated transcription of the human fetal (γ) globin gene (G. A. Partington et al., EMBO J. 3:2787-92, 1984). These findings were quickly confirmed in vivo wherein it was shown that pharmacological doses of butyric acid greatly increased expression of fetal globin in adult chickens rendered anemic by injections with phenylhydrazine (G. D. Ginder et al., Proc. Natl. Acad. Sci. USA 81:3954-58, 1984). Selective transcriptional activation was again thought to be due to hypo-methylation of the embryonic gene (L. J. Burns et al., Blood 72:1536-42, 1988). Others speculated that histone acetylation, a known effect of butyric acid, may be at least partly responsible for increased fetal gene expression (L. J. Burns et al., EMBO J. 3:2787, 1984).
Over 50 derivatives of butyric acid have since been found to be effective in stimulating fetal globin production (S. P. Perrine et al., Biochem. Biophys. Res. Commun 148:694-700, 1987). Some of these include butyric acid salts such as sodium and arginine butyrate, α-amino-n-butyric acid (butyramide; CH3CH2CH2CONH2), and isobutyramide (CH3CH(CH3)CONH2). Although promising in pilot clinical studies, treated patients were unable to maintain adequate levels of fetal globin in their system. It was later determined that many of these forms of butyric acid had extremely short-half lives. Oxidation in the serum, clearance by hepatocytes and filtration through the kidneys rapidly eliminated these agents from the patient's system. With others, patients rapidly developed tolerance or metabolites of compounds had the opposite desired effect.
Several studies have focused on the mechanism whereby butyric acid and other small organic molecules have been able to stimulate fetal globin expression (R. Oliva et al., Nuc. Acids Res. 18:2739, 1990). Experiments with cells in culture have indicated that butyric acid may act by increasing the level of histone acetylation by, possibly, decreasing the activity of one or more histone deacetylase. Resulting histone hyperacetylation may produce nucleosome unfolding and thereby increased gene expression. Other studies have indicated that hypo-methylation of the area of DNA around the β gene complex correlates with increased γ globin gene expression in thalassemic patients (S. Charache et al., Proc. Natl. Acad. Sci. USA 80:4842-46, 1983). Alternatively, butyric acid and other small molecules may function to increase specific gene expression by acting directly on agents which regulate transcription, the so-called transcription factors. These factors bind to sequence-specific sites along the genome at areas which control the expression of proximally located genes.
The present invention is directed toward overcoming one or more of the problems discussed above.